What Does Your Gut Do and How Does It Work? This 16-Point ‘Gut Guide’ Has the Answer

Your GI tract is an amazing place. From birth, it’s ground zero for a healthy immune system and controlling inflammation. The communities you have carving out their territory inside have a big impact on how you feel and function. Here are a few gut facts that explain how the magic happens, from making crucial nutrients to fighting cancer.

1. Totally Tubular

The digestive tract is one long tube from mouth to anus. What’s inside is technically outside the body until it’s absorbed through the intestine walls. Envision walking through one of those see-through underwater tunnels, but instead of a human being, it’s undigested pieces of fiber or protein or an E. coli bacterium wandering through. Your immune system is scanning everything in the tube to decide whether it’s OK to pass or if they need to mount an offensive to neutralize it.

2. It’s Crowded in There

About 100 trillion microbes — bacteria, viruses, fungi, and parasites — are living in your gut at any one time. That’s more than the number of red and white blood cells circulating in your bloodstream. If you were to weigh this treasure trove of critters, it would tip the scale at around five pounds.

3. Starring Roles

The main bacteria groups in the GI tract include Actinobacteria, Proteobacteria, Verrucomicrobia, Firmicutes, and Bacteroidetes. Lactobacillus are part of the Firmicute phylum and Bifidobacteria are part of the Actinobacteria phylum.

4. Information Superhighway

Gut bacteria may be sequestered away in the many undulating crevices of your colon, but they have a hotline to the central nervous system via neural, hormonal, and immune pathways. That’s why the makeup of your microbiome can affect what’s happening in your brain (such as depression, anxiety, and autism).

5. Working for a Living

Humans have a mostly synergistic relationship with their GI bugs. They help us digest certain foods, create energy, produce vitamins, absorb minerals, metabolize steroids, and — hugely important — regulate the immune system. In return, we feed them. They produce short chain fatty acids (SCFAs) and “those can be used as fuel by the cells within the large intestine and they also get absorbed through the cell wall and used in other parts of the body,” says Julie Stefanski, MEd, RDN, a certified diabetes educator in York, Pennsylvania, and spokesperson for the Academy of Nutrition and Dietetics.

“That’s one of the biggest benefits. You keep the cells healthy by nourishing them.” This helps maintain homeostasis of bacteria inside the intestinal tract. Two of the key plant compounds these bugs transform are lignans (found in flaxseeds and many vegetables) and isoflavones (found in soy products). They may help protect against heart disease, certain types of cancer, osteoporosis, and more.

6. The Good Fight

The intestinal barrier, the by-products of good bacteria (those short-chain fatty acids) and the immune system help keep bad, or pathogenic, bacteria at the proper level. When those bad bugs get a foothold, they can contribute to allergies, inflammatory bowel disease, cancer, obesity, heart problems, and diabetes.

7. Border Patrol

The majority of the immune system resides in the intestines and specifically in tissue called gut-associated lymphoid tissue (GALT). The immune system here is like a training ground, teaching the body’s defenses how to recognize new pathogens and handle any compounds that manage to slip through the thin cell lining of the intestines. “Fiber nourishes the GALT tissue so the nutrient is a big part of immunity,” says Stefanski.

8. Following the Food

Many of the bacteria in the gut are specialists, tackling primarily carbs or protein, for example. The microbiome can change quickly — even within 24 hours — when you shift your diet. If you start eating more meat or more plants, the populations of bacteria change to adapt with it, and that alters the entire microbiome. That’s how making healthy dietary adjustments affects your health, through the cascade of changes that happen as the bacterial landscape morphs.

9. Know Your P’s

Prebiotics are “a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon…improving the host’s health,” according to a review in the International Journal of Molecular Sciences. In short: Prebiotics feed probiotics. “A prebiotic fiber can’t be digested but it can be fermented, which produces SCFAs,” says Stefanski. Similarly, “probiotics are living non-pathogenic organisms used as food ingredients to benefit the host’s health.” Probiotics, which get added to the commensal bacteria already residing in the GI tract, are used to treat a variety of conditions, including multiple sclerosis, hypertension, atopic dermatitis, inflammatory bowel disease, and more.

10. Baby Talk

Infants get a head start on building their microbiome as they move through the birth canal and during breastfeeding. But that’s only one aspect that affects a newborn’s GI tract. The baby’s genes, the mother’s diet while she was pregnant, and diet and lifestyle after birth will also play a role, so if a C-section and/or breastfeeding aren’t in the cards, don’t panic.

11. Beat Bloat

The pain, gas, and bloating you occasionally experience may be coming from an imbalance of good and bad bacteria. In some studies, the presence of certain organisms, such as one called archaea, was associated with complaints of bloating or pain while the presence of bacteria, including bifidobacteria, was associated with less abdominal discomfort.

12. A Gift That Keeps Giving

Doctors can take fecal matter from one person and implant it into the colon of someone
else to promote the growth of healthy bacteria. In mice studies, researchers have transplanted “obese” or “lean” bacteria into the rodents and watched how the new bugs impacted the mices’ weight. That’s cool, but fecal transplants are mainly used to treat a serious condition called chronic C. difficile colitis, which is caused by the bacteria Clostridioides difficile.

13. The Diabetes Connection

Gut health — both the integrity of the intestinal lining and bacterial populations — is clearly associated with Type 1 and 2 diabetes. The presence of certain bacteria can increase or reduce insulin sensitivity and blood sugar control.

14. Attack of the Wrong Bacteria

Heart disease and the microbiome are closely related as well. Higher levels of pathogenic bacteria are associated with more intestinal permeability (leaky gut) and raised levels of inflammation. In addition, bacteria can synthesize cholesterol and other compounds that help stiffen and clog arteries.

15. Plants Rule

The immune response is important for fighting cancer and it should come as no surprise that your gut bugs affect that as well, especially stomach, colon, and prostate cancer. In one study, people with colon cancer had lower levels of a bacteria usually found in those who eat a plant-based diet.

16. The Secret to Losing Weight

There’s a strong association between GI tract bacteria populations and obesity. According to the International Journal of Molecular Sciences, a higher ratio of Firmicutes to Bacteroides is linked with a tendency to obesity. In addition, the short chain fatty acids produced by bacteria (think of it as bug “gas”) can also affect obesity, in part by regulating the brain-gut axis. That’s how far-reaching the activity in the gut is! In the future, oral medications containing bacteria may help control weight gain, even in certain parts of the body, like the belly or liver.

Good Gut Foods

Tea: Polyphenols from the Camellia sinensis plant can thwart the growth of bad bacteria in the gut. High in antioxidants and other healthy plant compounds, tea is often consumed with meals for a reason: The polyphenols in black tea, in particular, make a beeline to your digestive tract.

Yogurt: A fermented milk product, yogurt is rich in probiotics, organisms that have health benefits in the gut, specifically due to the “post” biotics (SCFA) that they create. Yogurt also contains conjugated linoleic acid, which may have a role in controlling age-associated weight gain.

Cruciferous Veggies: Broccoli, cauliflower, kale, cabbage, Brussels sprouts — these vegetables from the Brassica family are antioxidant all-stars, meaning they fight those free radicals that can trigger more inflammation. They’re also rich in fiber, indoles, carotenoids, and polyphenols.

Onions & Garlic: There’s a reason these two vegetables from the Allium genus are the base of so many different dishes. They’re prebiotic foods and they contain the polyphenol quercetin, which may have beneficial effects on the circulatory system (blood vessels) as well as the GI tract.

Oats: They not only provide food for bacteria, they can also help lower your cholesterol, thanks to their combination of soluble and insoluble fiber. The former blocks absorption of cholesterol into the blood stream. Instead, it buddies up to it and shows it out of the body in the feces.

Legumes: Beans, peas, lentils, and soybeans contain resistant starch and galactooligo-saccharides, which feed hungry bugs in the large intestine. The latter also help support the immune system. A large 2017 study, published in The Lancet, found that those who ate the most legumes had a lower risk of dying early.

The 5 Worst Foods for Inflammation

Saturated Fat: Eating a ton of meat and cheese often means you’re getting too much saturated fat. That’s not only harmful to the heart, it encourages the growth of bad bacteria (downing too much protein can do this as well), which can throw the whole microbiome out of balance.

Dairy: This cuts two ways. First, according to an analysis of 52 trials, published in Critical Reviews in Food Science and Nutrition, dairy was inflammatory mainly in those who are allergic to cow’s milk (shocker). In everyone else, it had anti-inflammatory abilities (unless it’s in excess).

Added Sugar: Eating too many sweets stokes inflammation and sets you down a path toward insulin resistance, diabetes, heart disease, weight gain, and other health problems. Per the American Heart Association, aim for no more than 25 grams of added sugar a day (the limit is 36 grams for men).

Additives and Preservatives: Many of these artificial ingredients alert the body’s defenses, creating inflammation where there didn’t need to be any. In addition, these chemicals can alter gut bacteria, reducing beneficial bugs and sending an invite to harmful critters.

Alcohol: A cocktail here and there is probably no problem — some studies have shown anti-inflammatory effects for alcohol — but regular alcohol consumption triggers inflammation via a compound called lipopolysaccharide (LPS). Researchers believe LPS signals for the release
of inflammatory cytokines and may contribute to obesity.

A version of this article appeared in our partner magazine, The Complete Guide to Anti-Inflammation.